The
SR-50A was introduced shortly after the SR-50 to to reduce manufacturing costs. Later
during production, probably with the introduction of the SR-56,
the SR-50A was revised again. The hardware of both the SR-50A and SR-51A
was redesigned to accommodate the TMC0501 Arithmetic chip, one (SR-50A) or two Scanning Read
Only Memories (SR-51A and SR-56) and none (SR-50A and SR-51A) or one
MULTI-REGISTER chip (SR-56).

Basically the same printed circuit board (PCB) was used
with different chip populations to manufacturer 3 different calculators priced
between US$ 109.50 and US$ 224.95.